The present invention concerns sol-gel alumina abrasive grits and particularly abrasive grits that are intended for incorporation in bonded abrasives.
Sol-gel alumina abrasive grits, particularly those that have an alumina crystal size that is essentially sub-micron are extremely effective in many applications especially when incorporated in vitreous bonded abrasive wheels. In the production of such wheels, the grits are mixed with a vitreous bond material and various organic temporary binders, burnout materials and/or lubricants and the mixture is placed in a mold. The temporary binders and/or lubricants are needed to facilitate the mixing and molding operation and the burnout materials are needed to ensure that the wheel has a desired degree of porosity when completed. The burnout material is of course intended to be completely removed during the firing of the wheel.
During the production process the temperature of the mixture is raised to the point at which the bond components mix, (if a raw bond is used), and flow until the abrasive grits are coated with the vitreous bond material and the molten bond forms a bond post connecting the points on adjacent grits that contact or are in close proximity. After allowing the maturing of the bond post structures, the temperature is allowed to ramp down until the wheel can be removed from the mold.
The temperature at which the bond is fired is rather critical because it is found that sol-gel alumina abrasive grits with alumina crystal sizes of the order of one micron or less are deteriorated by exposure to temperatures higher than about 1150.degree. C. for the prolonged periods required for formation of a vitreous bonded wheel. This is because the very fine crystal structure, which is closely connected to the excellent grinding performance, begins to be coarsened. For this reason it is strongly recommended that, when using sol-gel alumina abrasive grits, particularly with seeded sol-gel alumina abrasive grits, a bond should be used that is matured at a temperature below 1150.degree. C. and preferably below about 1000.degree. C.
In addition, the higher the temperature at which the bond is formed, the greater the extent of the penetration of the abrasive grits by silica from the vitreous bond system. This interaction, while not a serious problem with low temperature-matured bonds, is something that needs to be considered in developing optimum performance.
It is found however that, using such low temperature-matured vitreous bond materials, the organic materials that are supposed to be completely burned out in fact leave a residue of carbon. This residue collects in surface pores on the grits, which are typically white or colorless, such that the wheel produced may have black spots.
A process has now been developed that permits the use of low temperature vitreous bonds with sol-gel alumina abrasive grits without the development of the unsightly blemishes, or black spots, on the surface of the vitreous bonded product.
The process has also been found to have a beneficial effect on reducing the interaction between the bond and the abrasive grits thereby allowing the development of stronger bonds.